Framework construction

ABSTRACT

A novel framework construction, e.g., a ladder, is provided. It includes at least two longitudinal beams disposed in substantially parallel alignment. Each beam is of a novel construction having a cross-section in the form of a pair of legs joined by a web, the web including a central area of substantially uniform thickness, and a pair of lateral areas, each such lateral area being of gradually increasing thickness in a direction away from the central area. A plurality of struts is provided, the struts being disposed transversely between, and connected to, the beams, the struts further forming an interlocking connection with the web. A novel procedure for producing this framework construction is also taught.

I United States Patent [191 Engvall et a1.

.1 Dec. 24, 1974 FRAMEWORK CONSTRUCTION [75] Inventors: ErikArne Engvall, Vasteras,

Sweden; Thomas Ritscher, Adliswil,

2 11 Appl. No.: 307,503

[30] Foreign Application Priority Data Nov. 23, 1971 Switzerland 17018/71 [52] US. Cl. 182/194, 182/228 [51] Int. Cl. E066 l/02, E06c 7/08 [58] Field of Search 52/729; 182/228, 194, 46

a [56] References Cited 1 UNITED STATES PATENTS 426,558 4/1890 Dithridge 52/729 1,360,720 11/1920 Brown r 52/729 1,495,570 5/1924 Blakeley 52/729 1,768,833 7/1930 Edwards 52/729 1,786,938 12/1930 Edwards 52/729 1,990,155 2/1935 Young 52/729 2,589,304

Spangler 52/729 2,855,134 10/1958 Arnold ..182/46 2,966,229 12/1960 Frezieres.. 182/46 3,241,285 3/1966 Baroni r 52/729 FOREIGN PATENTS OR APPLICATIONS 611,668- 11/1948 Great Britain .1 52/729 1,900,643 8/1970 Germany 182/228 Primary Examiner-Reinaldo P. Machado Attorney, Agent, or FirmMichael Klotz V [57] ABSTRACT A novel framework construction, e.g., a ladder, is provided. It includes at leasttwo longitudinal beams disposed in substantially parallel alignment. Each beam is of a novel construction having a cross-section in the form of a pair of legs joined by a web, the web including a central area of substantially. uniform thickness, and a pair of lateral areas, each such lateral area being of gradually increasing thickness in a direction away from thecentral area. A plurality of struts is provided,

the struts being disposed transversely between, and connected to, the beams, the struts further forming an interlocking connection with the web. A novel procedure for producing this framework construction is also taught.

21 Claims, 6 Drawing Figures FRAMEWORK CONSTRUCTION BACKGROUND OF THE INvENTIoN substantially parallel alignment and mutually interconnected by means of a plurality of struts, the struts preferably being "arranged in a mutually equidistanced relation. Examples of such framework constructions include scaffoldings, railings, guard-rails, supports for electrical cables, ladders, etc. r

b. Description of the Prior Art The beams of such prior art framework constructions generally have been made of different structural materials, such as, for example, wood or metal and usually have had a generally circular or rectangular crosssection. For metal beams it was conventional to use profiles or cross-sectional shapes resembling an I, an H or double T. In such beam profiles, the cross-sectional shape generally'comprised two juxtaposed legs interconnected by means of a cross-piece or web. Conventional H beams ordouble T beams comprise a'pair of legs, i.e., the vertical legs of the H, and a web, i.e.,'the horizontal cross-piece of the H. In cross-section (normal to the length of the beam), the web of a conventional I-I-beam predominantly is of substantially uniform widthJMany prior art framework constructions, such as, for example, ladders are made of structural light metals or metal alloys (e.g., of aluminum) with .beams having such a profile. These provide satisfactory bending stiffness and torsion stability even when the beams have relatively small cross-sectional areas. It was noted, however, that the side stiffness generally was unsatisfactory. This latter requirement is important not only for ladders, particularly light ladders of some length, but also for-other types of framework constructions of the types mentioned above where the load is not acting predominantly in the longitudinal direction of the beams, i.e., not predominantly in adirection per.- pendicular to thestruts, for example, in the case of guard-rails, railings, etc. In any of these cases, the use of relatively light, high grade and hence relatively expensive structural materials, such as, for example, aluminum alloys, calls for framework constructions which have a minimum weight yet have an optimum balance of bending and torsion stability when subjected to loads in directions which may be either parallel to, or perpendicular to, the longitudinal direction of the beams.

Attempts to increase the bending and torsional stability by simply increasing the cross-sectional area of the beams in general or in specific areas of stress concentrations generally did not provide for a sufficient increase of bending and-torsion stability to justify the resulting increased weight and the increased costs of the SUMMARY OFTHE INVENTION a. Aims of the Invention It is therefore an object of this invention to provide a novel beam structure and novel framework construction capable of general mechanicalstability, with surprising improvements of side bending and torsion stability of light metal framework constructions, e.g.,

structures of the type set forth above made of alumiprovements in side bending and torsional stability thereof.

Yet another object of this invention is the provision of a novel framework construction including a novel beam configuration and a plurality of struts cooperating therewith to provide increased mechanical stability and improvements in side bending and torsional stability thereof.

A still further object of this invention is the provision of a novel ladder structure having increased mechanical'stability and improvements in side bending and tor- .sional stability.

Another object of this invention is the provision of a novel procedurefor producing such novel framework construction. 1 I

b. Broad Statement of the Invention By this invention, then, a beam is provided, having a cross-section in the form of a pair of legs joined by a web, wherein theweb includes a central area of substantially uniform thickness, and a pair of lateral areas, each such lateral area being of gradually increasing thickness in a direction away from the central area; and further wherein (i) the ratio of a thesum of the length of the two lateral areas and the lengthof the central area, to b the length of the central area is 4:1, and (ii) the ratio of a the sum of the length of the two lateral areas and thele'ngth of the central area, c the sum of the length of the two. lateral areas is 2.521.

Bythis invention, a framework construction is also provided, comprising at least two longitudinal beams disposed in substantially parallel alignment and a plurality of struts transversely disposed between and interconnecting the beams, each of the beams having a cross-section in the form of a pair of legs joined by a web, the web having a central area of substantially uniform thickness, and a pair of lateral areas each such lateral area being of gradually increasing thickness in adirection away from the central area, the struts forming an interlocking connection with the web.

By this invention,a ladder is also provided comprising two longitudinal beams made of an extrudablelig ht metal alloy disposed in substantially parallel alignment and a plurality of equidistanced hollow struts interconnecting the beams, and made of an extrudable light metal alloy arranged between and substantially normal to the beams and having a generally rectangular crosssection, each of the beams havinga generally H-shaped cross section in the form of a pair of legs joined by a web, wherein the web includes a central area of substantially uniform thickness, and a pair of lateral areas, each such lateral area being of gradually increasing thickness in a direction away from the central area; the

central area of the web having a'plurality of openings to receive opposite end portions of the struts; each strut 'end portion beingprovided with a pair of flanges formed wedgingly to interlock with the web of the beam adjacent to suchend portion of such strut.

In addition, by this invention, a process is provided for producing a framework constructi on. Theprocess includes three main steps. The first step comprises arranging (A) at least two longitudinal beams made of an extrudable light metal alloy in a substantially parallel alignment, such beams having a cross-section in the form of a pair of legs joined by a web, wherein the web other side face constitutes an outer web face, the central area of the web being provided with a plurality of generally rectangular openings, and (B) a plurality of struts made of an extrudable light metal alloy and having a substantially rectangular crosssection, each strut being provided with a pair of first flanges, each of such first flanges being arranged near one end of an associatedstrut and comprising an inner face to contact the inner web face,.and with strut ends capable of being inserted into the openings in the web and extending therethrough. The second step comprises assembling the beams with a plurality of such struts so that'each of the first flanges faces an inner web face while the strut ends extend through the openings. The third step comprises pressingly deforming the strut ends extending through the web openings to form second flanges at the outer web faces and to hold each of the web center portions between pairs of first and second flanges, each pair of flanges being wedgingly locked by the lateral areas of the web.

c. Variants of the Invention According to embodiments of this invention, as described above, the web of the beam for the framework construction has a flat center portion and two tapered side portions such that the thickness of the web increases in the directions away from the central area of the web. 1

It is further noted that the web of the beams for the framework constructions according to aspects of this invention can be described as a double Y in which the forks extend away from the point where the Ys are joined. It' is to be emphasized'that this doubly-tapered shape relates to the web portion of the beams which further include the legs.

The framework construction according to an embodiment of this invention comprises at least two such beams disposed in substantially parallel alignment. The

beams are arranged in a common plane and the angle between them should be zeroor not more than about Also, the term substantially parallel alignment is intended to indicate that the upper and lower legs of the beams are arranged in common upper and lower planes (viewing the cross-sections as double Ts) while the webs are in substantially parallel planes.

Each web has a central area and a pair of lateral areas. The term lateral area defines those portions of the web between the central area of the web and the areas where the web joins the legs. Each lateral area increases in thickness toward the adjacent leg of the beam. Preferably, each lateral area ofa web constitutes at least about 20 percent of the entireweb length which web length consists of the length of the pair of lateral areas and the length of the single central area.

The increase of the thickness of the lateral areas of the web generally is a continuous increase. This can be either a linear or a geometric increase, as will be explained in more detail below. Accordingly, the outer faces of each of the lateral areas of the web can be said to form a wedge. It is generally preferred that the wedge angle, i.e., twice the inclination of the outer face against the web axis (or the plane of the web center portion) be in'the range of from about 10 to about 35,

. a range of from about 14 24 being particularly preferred for the wedge angle.

In one variant of this invention, it has been found that the struts which transversely interconnect the beams should form an interlocking connection with the webs of the beams. The profile or cross-sectional configuration of the web as described above should cooperate with the contacting face or faces of the struts in such a manner as to lock the struts in their position. As will be seen below, the double wedge of the lateral areas of the web can be used further to increase this lateral in-' I terlock by providing the struts with flanges, preferably two flanges at each strut end of the type which can be produced by crimping the wall of a hollow metal column with a circular or, preferably, rectangular crosssection.

In a most preferred embodiment of the invention, the struts are hollow members having a substantially rectangular, and preferably a substantiallysquare, crosssection, the webs being provided with substantially rectangular openings to receive the strut ends. If the strut portions near each end of the strut are provided with a first flange or crimp of sufficient size (the distance between the two parallel sides)-substantially to cover the length of the central area of the web, the wedge-shaped sides of the web provided by the lateral areas will act as an interlock or keying means for the flange edges which are parallel with the beam direction. In addition, if the web center portion is held tight between two such flanges of a strut end, a very small overlap of the flange over the central area of the web into the lateral areas of the web will produce an additional increase of the strength of the joint between struts and beams, particularly if well known techniques of crimping or press-shaping light metal alloys are used to produce the interlocked and wedged connection of the strut flanges and the webs of the beams.

As noted above, the term wedge as used to describe the form of the lateral areas of the web includes both linear as well as non-linear increases. One example of a non-linear increase embraced in this invention is that of a logarithmic spiral. In such embodiment, both sides of each lateral area of the web are curved oppositely in this manner and, as will be seen below, such logarithmic curvature or at least a shape which approaches this form can extend into the legs of the beam to the point where the curve turns back toward the center portion.

BRIEFDESCRIP'TION OF THE DRAWINGS tive view of an embodiment of the novel beam accord ing to one aspect of this invention;

' view of a lateral area of the web andv of the adjacent leg of anotherembodiment of the novel beam according to one aspect of this inv ention', I j 1 FIG. 3 is adiagrammatic, fragmentary, elevational view of another embodiment of the lateral area of the web and of the adjacent leg of another embodiment of the novel beamaccording to one aspect of this invention; j 3

FIG. 4 is a schematic diagram of the general formof an I-I-beam showing dimensional relations of preferred beams according to an aspect of this invention;

FIG. 5 is a diagrammatic, fragmentary, sectional view illustrating an embodiment of the interlocking relation of beam and strut in a construction according to another aspect of this invention; and

FIG. 6 is a fragmentary, sectionalview illustrating the interlocking relation of beam and strut in a preferred construction according to another aspect of this invention. i

DETAILED DESCRIPTION OF THE DRAWINGS a. Description of FIG. 1

Turning firstly to FIG. 1, beam 11 comprises a web 12 consisting of a central area 13 and a pair of lateral areas 14, 15. The central area 13 of the web 12 is of a substantially uniform thickness while each of the lateral areas 14, 15 increases in thickness from the central area 13 of the web 12 to the associated adjacent legs 17, 19. In other words,'the lateral areas 14, 15 of the web .12 are. generally inwardly tapered from the area adjacent legs l7, 19 towards the central area 13 of the web 12. The taper shown in FIG. 1 is somewhat exaggerated for better understanding, showing, for example, a taper of 40.-As will be explained in greater detail below, the preferred taper is from 10 to 35, with a taper of 14 24 being the preferred range.

The centralarea 13 of the web 12 is provided with a plurality of substantially rectangular openings 16, of which three are shown in the drawing. It is understood, of course, that the actual number of openings, their distances, shape and size will depend upon the required number of struts per unit of length of a given construction, the total length of the construction and the crosssectional shape of the struts. The square configuration of openings 16 is-a preferred embodiment.

Legs 17, 19 adjacent to the lateral areas 14, 15 of the web 12 can be shaped as shown, i.e., with leg ends 18, 20 (respectively) bent inwardly to face one another toward the central area 13 of the web 12, but other'configurations, for example, arcuate, straight, etc., will be suitable as well. In any case, the exact form of the legs l7, 19 is not a critical feature.-

. A framework construction according to one aspepct of this invention comprises at least two substantially similar beams of the type described above, disposed in substantially parallel alignment. In one practical embodiment, namely for ladders, a pair of such beams 11 will be arranged with the inner side face 131 of the web portion 12 in face-to-face juxtaposition with the corresponding inner side face 131 of the web portion of the struction, for example, for cable supporting structures,

etc.

Preferably, the beams 11 are made from an extrudable light metal, for example, an aluminum alloy of the type which will be discussed in greater detail below.

b. Description of FIG. 2

Turning now to FIG. 2, the cross-sectional shape of the tapered lateral portion 6 of the web 12 is shown for a non-linear increase of the thickness thereof from the area indicated as 4, i.e., the end of central area 20f the web 12, towards its associated adjacent leg 3. Faces 1a, lb of lateral area 6 generally follow the form of logarithmic spirals extending to the inwardly bent ends 5a, 5b of leg 3. While the shape of a logarithmic spiral is wellsuited for the lateral areas of the beam, various modifications of the ideal curvature in accordance with such parameters as beam dimension, strut dimension and properties of the alloy will be apparent to the expert. In any case, a curved shape of the side faces la, 1b of lateral area 6 of the web is not critical in any aspect of this invention. It is apparent that FIG. 2 shows only a portion of a beam profile. V

c. Description of FIG. 3

Turning now to FIG. 3, a portion of a beam crosssection which can be considered to be an approximation to the-cross-sectional shape shown in FIG. 2 is shown. The modification in FIG; 3 is essentially due to practical requirements of producing such beams, for example, by extruding light metal alloys'through a die. In FIG. 3, the transition from leg 31 to the lateral area 22 of the web starts with a curvature generally indicated as42, and proceeds via a substantially planar portion 43 to transition point 41, which is the boundary between the lateral area 22 of the web and the central area 21 of the-web. Central area 21 has a length indicated by S The general shape or taper of the lateral areas (6 in FIG. 2 or 22 in FIG. 3) can be said to be wedge-shaped" and is characterized by a wedge angle which is twice the angle 0: shown in FIG. 3. In practical embodiments and in particular in the embodiments described with reference to FIGS. 5 and 6 below, the

wedge angle (2 X a) generally will be about 10 and about 35, preferably between about 15 and about 30 and most preferably between about 14 and about 24",

d. Description of FIG. 4

Some preferred dimensional ratios of beam profiles of central area of the web, of lateral areas of the web, and of length of leg of one aspect of this invention will now be described. by means of the diagram shown in FIG. 4.

Preferably, the ratio of total length (length S of the web to the length of the central area 21 (length S of the web will generally be in the range of from about 2.5 to l,'prefe'rably in the range of from about 2.2 to 1.2. The thickness of the central area 21 of the web is substantially uniform and the increase of thickness in the lateral area 22 (length S of the web (which as explained above can be said to be wedgeshaped) will. start at a pointn (S indicated as 50.

' Generally, both lateral areas 22 (length S )'will have thesame length. Also, it is preferred that the ratio of the total length of web (8 to the sum total of the lengths of the two lateral areas, in other words, the ratio S :2S will be between 4 and l, and preferably will be between 3.4 and 1.9. The ratio of the total length of the web (length 5 to the lengths of legs 3l 7 (length B ),',i.e., the ratio S :B,,, preferably is in the rangeof from about 2.0 to 1.0, more preferably between about 1.9 and 1.4. The ratio of the total length of the web (length S to the thickness (thickness S about 0.04 to 0.02.

The ratio of web width (width S to the width (width 3 of legs 31 generally is between about 2.0 and about 0.5, and preferably is between about 1.7 and about 0.6, a lower limit of .1 being suitable for most purposes.

Beams having the above-noted values show unexpected utility inrespect of providing framework constructions of increased mechanical stability with surprisingimprovements of side bending and torsional stability when compared to beams having the crosssectional configuration shown in French Pat. No. 2,044,017 dated Feb. '8, 1971 in the name of A. Rigoine de Fougerolles. I

The preferred ratios set forth above in connection with FIG. 4 are of particularadvantage for beams made by extrusion from extrudable light metal alloys on an aluminum basis. Preferably, the struts are made of similar materials and can also be extrudates of the type known per se. Typical examples of such alloysare AIM- gSi alloys as specified in UnitedStates Standards No.

6351/6 or the corresponding Swedish Standards No. 4212-6. Generally, the strength (tensile) of the aluminum alloys used for beams and struts should be at least about 25 kiloponds per mm (substantially corresponding to a Webster B hardness of at least about 16). Preferably, the strength of the alloy should not be more than about percent below the values justindicated.

As previously mentioned, the dimensions of the strut are correlated with regard to the length S of the central area of the web. In general, the central area of the web will have substantially the same length as or be somewhat smaller than, the side-to-side distance of the contacting face of rectangular struts. The term contact face refers to that part of the ends of the struts which contacts the web side of the beam. Generally, this contact face will be part of a flange portion of the strut, e.g., a crimped bulge or rim formed around the strut periphery and usually will be similar to but somewhat larger than the cross-section of the strut.

In general, the preferred cross-sectional shape of the struts is substantially rectangular, with a square crosssectional shape constituting an even more preferred embodiment. The edges may be rounded as required.

- 6 2 and an outer flange 63 firmly engaging the central area 65 of the web with outer ends 621, 631 of flanges 62, 63 (respectively) in contact with lateral areas 641, 642.

In practice this can be achieved, as illustrated in FIG. 6. FIG. 6 shows a beam and a strut 48. The beam includes a web having a central area 52, and a pair of tapered lateral areas 46. The tapered lateral areas meet an associated leg 45, and each leg 45 has each of its ends bent inwardly as at 47 to face-one another towards the central area 46 of the web. An end portion 481 of strut 48 is provided with acrimped rim 49 which is to become the inner flange. Then the remaining free end will be inserted into an opening (16, FIG. 1) in central area 52 (13, FIG. 1) of the web (12, FIG. 1) to extend therethrough. Now, a second crimped rim 51 is formed at the end portion 481, e.g., by compression and in accordance with the deformation capacity of the material used. By supporting crimped rim 49 against a pressure laterally applied at the end portion 481, crimped rim 51 is formed and both rims 49, 51 are both pressed against the central area 52 of the web. In this way, an extremely strong interlocking connection between strut 48 and the web of the beam can beproduced.

It is generally preferred to compress and deform rims 49, 51 to such an extent that the flanges produced 7 thereby extend to the flared lateral areas 46 of the web so that a wedge interlock will be formed due to the increasing thickness of such lateral areas 46. With extrudable light metals of the type mentioned above, the interlocking connection between strut and beam produced in this manner will markedly contribute to the bending and torsion stability of the strut construction. One manner of performing the above-described interlocking connection between beam and srut is described in British Pat. No. 1,229,391 dated Apr. 21, 1971 in the name of A. B. Carl Keijser &'Co.

It is to be understood that the strut/beam interlock shown in FIGS. 5 and 6 for one side of the construction will be repeated, or produced simultaneously, at the other juncture or other junctures, asthe case may be. In the production of ladders, it is preferred to produce the interlock at both ends of each strut simultaneously.

Comparative tests of framework constructions accordingto an aspect of this invention,,e.g., as shown in FIG. 6, with framework constructions of the same basic structure but with a conventional beam (i.e., having no increasing thickness of the lateral areas of the web) show remarkable increases of:

a. the bending strength (measuring the deformation of a construction consisting of two substantially parallel beams and a plurality of struts interconnecting the beams. at regular distances with a test load applied on both beams);

b. the side bending strength (structure as above, the

load being applied on one beam only); and

0. general improvements of the torsion stability. For example, the side bending strength of the construction according to one embodiment of this invention was about eight times higher than that of the comparative structure. Generally, the test methods employed were those specified in Danish Industrial Standards (DS 2069.0) which are considered to constitute the most severe tests among accepted oflicial standards for such framework constructions.

While the above description relates mainly toframe work constructions for ladders made of extrudable light metal alloys, it is understood that many variations with regard to the use of the structures, the materials employed for their production, the specific forms, dimenbeams against the strutsof one of these beams and the next beam. Furthermore, other structural materials can be used for both beams and struts, including synthetic posed contact faces cooperating clampingly to hold opposite side faces of said central area of said web; and

polymers, e.g., duroplasts either unreinforced or reinforced with substances such as, for example, glass fibers. I

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be,

within the full range of equivalence of the following claims.

l claim:

l. A ladder comprising two longitudinal beams made of an extrudable light metal alloy disposed in a substantially parallel alignment, said beams having a generally l-l-shaped cross-section inth'e form of a pair of legs joined "by a web, wherein the web includes a central area of substantially uniform thickness, and a pair of lateral areas, each such lateral area being of gradually increasing thickness in a direction away from said central area; the central area of the web having a plurality of openings therein; a plurality of equidistanced hollow struts made of an extrudable light metal alloy arranged substantially normal to and interconnecting said beams and having a generally rectangular cross-section; and opposite end portions of said struts being received in the openings in said web, each strut end portion being providedwith a pair of flangesformed wedgingly to interlock with the central area and abut a portion'of said lateral areas of the web of the beam adjacent said strut end portion.

2. A framework construction comprising at least two longitudinal beams disposed in substantially parallel alignment, each said beam being in the form of a pair of legs joined by a web, said web including a central area of substantially uniform thickness, and a pair of lateral areas, each said lateral area being of gradually increasing thickness in a direction away from the central area; and a plurality of struts, said struts being disposed transversely between and the ends of said struts being connected to said beams, said ends of said struts forming an interlocking connection with the central area and abutting a portion of said lateral areas of said web.

3. The construction of claim 2, wherein said struts have a substantially rectangular cross-section; and wherein said central area of said web of said beams is portions of said struts, each strut end portion being procontact faces having two opposite edges wedgingly abut with said lateral areas of said web.

5. The construction of claim 2 wherein the total length of said lateral areas comprises at least about 20 percent of the total length of the web.

6. The construction of claim 5 wherein each lateral area of said web increases in thickness in a general wedgeJike mannerwith opposite faces of each of said lateral areas of said web including an angle of between about 10 and about 7. Theconstruction of claim 6 wherein said angle is from about 14 to about 24.

8. The construction of claim 2 wherein said legs of said beam have their free ends bent inwardly.

9. The construction of claim 8 wherein said inwardly bent leg ends comprise a bead-like end.

'10. The construction of claim 2 wherein the ratio of the width of the central area of said web to the general thickness'of the legs is in the rangeof from about 0.6 to about 1.8.

11. The construction of claim 2 wherein the ratio of the total web length to the length of the legs is in the rangeof from about 1.4 to about 1.9.

12. The construction of claim 2 wherein the ratio of the thickness of said centralarea of said web to the vided with. at least one contact face shaped to fit into Said central area of said web and wedgingly to abut with said lateral areas of said web.

4. The construction of claim 3 wherein each end portion of said struts comprises: two face-to-face juxta- 13. The construction of claim 2 wherein said struts are hollow members made of an'extrudable light metal alloy and wherein said struts have a generally rectangular cross-section.

14. The construction of claim 13 wherein said metal alloy is aluminum and wherein said struts have a generally rectangular cross-section.

15. The construction of. claim 2, wherein (l) the ratio of (a) the sum of the length of the two lateral areas and the length of the central area to (b) the length of the central area is about'4 to l; and (ii) the ratio of (a) the sum of the length of the two lateral areas and the length of the central area to (c) the sum of the length of the 'two lateral areas is about 2.5 to l.

17. The construction of claim 15 wherein the ratio of the width of the central area of said web to the general thickness of the legs is in the range of from about 0.6 to about l .8.

18. The construction of claim 17 wherein said beams are made of an extrudable light metal alloy.

19. The construction of claim 18 wherein said beams are made of an aluminum alloy.

' 20. The construction of'claim 15 wherein the ratio of the total web length to the length of the legs is in the range of'from about 1.4 to about 1.9.

21. The construction ofclaim 15 wherein the ratio of the thickness of said central area of said web to the total length of said web is from about'0.04 to about 

1. A ladder comprising two longitudinal beams made of an extrudable light metal alloy disposed in a substantially parallel alignment, said beams having a generally H-shaped cross-section in the form of a pair of legs joined by a web, wherein the web includes a central area of substantially uniform thickness, and a pair of lateral areas, each such lateral area being of gradually increasing thickness in a direction away from said central area; the central area of the web having a plurality of openings therein; a plurality of equidistanced hollow struts made of an extrudable light metal alloy arranged substantially normal to and interconnecting said beams and having a generally rectangular cross-section; and opposite end portions of said struts being received in the openings in said web, each strut end portion being provided with a pair of flanges formed wedgingly to interlock with the central area and abut a portion of said lateral areas of the web of the beam adjacent said strut end portion.
 2. A framework construction comprising at least two longitudinal beams disposed in substantially parallel alignment, each said beam being in the form of a pair of legs joined by a web, said web including a central area of substantially uniform thickness, and a pair of lateral areas, each said lateral area being of gradually increasing thickness in a direction away from the central area; and a plurality of struts, said struts being disposed transversely between and the ends of said struts being connected to said beams, said ends of said struts forming an interlocking connection with the central area and abutting a portion of said lateral areas of said web.
 3. The construction of claim 2, wherein said struts have a substantIally rectangular cross-section; and wherein said central area of said web of said beams is provided with a plurality of openings to receive end portions of said struts, each strut end portion being provided with at least one contact face shaped to fit into said central area of said web and wedgingly to abut with said lateral areas of said web.
 4. The construction of claim 3 wherein each end portion of said struts comprises: two face-to-face juxtaposed contact faces cooperating clampingly to hold opposite side faces of said central area of said web; and contact faces having two opposite edges wedgingly abut with said lateral areas of said web.
 5. The construction of claim 2 wherein the total length of said lateral areas comprises at least about 20 percent of the total length of the web.
 6. The construction of claim 5 wherein each lateral area of said web increases in thickness in a general wedge-like manner with opposite faces of each of said lateral areas of said web including an angle of between about 10* and about 35*.
 7. The construction of claim 6 wherein said angle is from about 14* to about 24*.
 8. The construction of claim 2 wherein said legs of said beam have their free ends bent inwardly.
 9. The construction of claim 8 wherein said inwardly bent leg ends comprise a bead-like end.
 10. The construction of claim 2 wherein the ratio of the width of the central area of said web to the general thickness of the legs is in the range of from about 0.6 to about 1.8.
 11. The construction of claim 2 wherein the ratio of the total web length to the length of the legs is in the range of from about 1.4 to about 1.9.
 12. The construction of claim 2 wherein the ratio of the thickness of said central area of said web to the total length of said web is from about 0.04 to about 0.02.
 13. The construction of claim 2 wherein said struts are hollow members made of an extrudable light metal alloy and wherein said struts have a generally rectangular cross-section.
 14. The construction of claim 13 wherein said metal alloy is aluminum and wherein said struts have a generally rectangular cross-section.
 15. The construction of claim 2, wherein (1) the ratio of (a) the sum of the length of the two lateral areas and the length of the central area to (b) the length of the central area is about 4 to 1; and (ii) the ratio of (a) the sum of the length of the two lateral areas and the length of the central area to (c) the sum of the length of the two lateral areas is about 2.5 to
 1. 16. The construction of claim 15 wherein the ratio (i) is about 3.4 to 1.9, and the ratio (ii) is about 2.2 to 1.2.
 17. The construction of claim 15 wherein the ratio of the width of the central area of said web to the general thickness of the legs is in the range of from about 0.6 to about 1.8.
 18. The construction of claim 17 wherein said beams are made of an extrudable light metal alloy.
 19. The construction of claim 18 wherein said beams are made of an aluminum alloy.
 20. The construction of claim 15 wherein the ratio of the total web length to the length of the legs is in the range of from about 1.4 to about 1.9.
 21. The construction of claim 15 wherein the ratio of the thickness of said central area of said web to the total length of said web is from about 0.04 to about 0.02. 